1. Field of the Invention
This invention relates to an improvement in a differential pressure switch which is responsive to the pressure of a stored gas, a control pressure, within the switch, and the differential between that pressure and the pressure of the environment in which the switch is maintained and which it is monitoring.
While not limited to such use, the improved differential pressure switch has especial utility in a vehicular expandable confinement or inflatable "air bag" safety restraint system. In what is commonly known as a hybrid system, certain safety restraint systems utilize, in addition to a gas generator for the generation of gas, a vessel which stores compressed gas to supply additional gas under pressure to protect the occupants of the vehicle upon impact thereof with a harm producing object.
2. Description of the Prior Art
The expandable confinement or inflatable bag has a collapsed, folded, inoperative condition for storage in a steering wheel, dashboard, or other location within a vehicle forward of a normal occupant position. Upon the occurrence of a collision of the vehicle, expansion of the inflatable bag may be effected or augmented by a fluid supply in the form of a pressure vessel defining a chamber which contains a supply of pressurized gas. The stored gas is released from the chamber by rupturing a portion of the pressure vessel which communicates with suitable gas directing means with the interior of the inflatable bag. The bag inflates in front of the occupant and acts as a protective cushion. The pressurized supply of gas is a very important part of the system. If the gas pressure falls below a predetermined level, the system will not operate.
Thus, in an inflatable bag safety restraint system that depends for its operation upon stored compressed gas, there is a need to monitor the pressurized vessel containing the stored gas to detect any leakage of gas.
By way of example and not limitation, it is noted that such pressurized vessel can be pressurized with an inert gas, such as argon, or a mixture of argon and another inert gas, to approximately 3200 psi at room temperature. Such vessel must be adapted to maintain the stored gas pressure so that the pressure does not drop by more than about 200 psi when at room temperature during the life of the vehicle which may be ten (10) years or more. Such limits are by way of example only and are not intended in any manner to place restrictions on the scope of the claimed invention.
It has been proposed in the prior art to provide means for detecting loss in the pressure in a pressurized vessel caused by means other than temperature change, such as a leak in the vessel or other damage thereto. The state of the prior art, in this connection, is indicated by the following U.S. Pat. Nos.:
______________________________________ U.S. Pat. No. Issue Date Patentee ______________________________________ 1,582,154 April 27, 1926 A. Zeiher et al. 3,723,684 March 27, 1973 R. Greenwood 3,735,376 May 22, 1973 J. M. Kermer et al. 3,760,350 Sept. 18, 1973 E. L. Johnson 3,818,764 June 25, 1974 J. P. Wagner 3,850,039 Nov. 26, 1974 H. G. Brakebill 3,859,845 Jan. 14, 1975 J. T. Sawyer 4,049,935 Sept. 20, 1977 W. P. Gruber 5,073,124 Dec. 17, 1991 F. R. Powell Russian 726603 Aug. 4, 1980 French 2 431 178 Aug. 2, 1980 J. Segall et al. ______________________________________
The pressure detecting means disclosed in the foregoing patents are characterized, in general, by their complexity.
U.S. Pat. No. 1,582,154 discloses an oil pressure-controlled indicator for motors comprising a flexible diaphragm, a pair of contacts mounted on and movable with the diaphragm and adapted to be moved into engagement with each other in response to failure of oil pressure.
U.S. Pat. No. 3,723,684 discloses a pressure responsive switch wherein axial force applied by a diaphragm responsively to pressure in a tank loads parallel flexible contact blades, causing them to bend apart. With decrease in tank pressure the loading of the contact blades is reduced. This causes them to move together to close a warning circuit.
U.S. Pat. No. 3,818,764 employs a hollow ribbon-like elongated bourdon tube which twists and untwists to operate an electric switch in response to a pressure differential across the wall of the tube.
In U.S. Pat. No. 5,073,724, positioned interiorly of each of a plurality of flexible tubular elements, normally out of contact with the conductive inner wall thereof, is a contact pin. Interconnection of the contact pin and the inner wall is effected by the application of external fluid pressure to the tubular element.
Russian patent 726603 discloses a pressure switch having inner flexible diaphragms forming a pressure chamber and outer movable contacts that close upon the application of pressure to the chamber. Applied pressure through a channel to the chamber deforms the diaphragms and the movable contacts, causing the latter to move apart and make contact with other fixed contacts in a series arrangement.
French patent 2 431 178 discloses an electric switch involving one or more non-metallic conducting membranes arranged to make contact with metal or other membrane electrodes responsively to shock, pressure, weight, etc., and operative for uses such as alarms, automatic door openers, etc.
In the others of the listed patents, fluids of a control pressure and of a pressurized vessel being monitored are separated from each other by a single flexible diaphragm. This necessitates the use of relatively complex and costly structural arrangements for sensing the deflections of the diaphragm resulting from changes in the differential pressure between the control pressure and that of the pressurized vessel.
Thus, there is a need and a demand for a simple, low cost differential pressure switch, of high reliability, which is responsive to the pressure of a stored gas, that is, a control pressure within the switch, and the differential between that pressure and the pressure of a pressurized vessel in which the switch is positioned, which latter pressure is being monitored.